Because telephone lines are often used to carry a variety of diverse signal formats, care must be taken when connecting a line to equipment such as a telephone test set, in order to ensure that the connection is non-intrusive and does not degrade the quality of signals being conveyed over the line. This requirement is particularly crucial in the case of connecting a device to a `data` line, especially a powered data line, namely, a line that not only may be transmitting digital data traffic, but also carries a DC voltage to be used for powering transmission devices along the line (for example repeater and customer premise equipment). If an external device, such as a test set, is connected to a powered data line in such a manner as to load the line (e.g. by placing a DC load impedance between either tip or ring and ground), severe degradation and/or interruption of service will occur.
Of course, simply connecting leads of a test set to the line, without effectively enabling circuitry functionality of the test set which would place a load on the line, is not necessarily intrusive. However, if a test set, once connected to the line by the craftsperson, is placed in a condition that would modify the electrical condition of the line, such as the test set going OFF-HOOK and thereby inserting a loading impedance across the line, any digital data traffic carried by the line is impacted, which creates not only an unacceptable disruption in information transmission to customers, but an embarrassment and loss of revenue to the telephone service provider.
Previous attempts to deal with this problem have included the use of compact or `pocket` data traffic detector devices, which are essentially frequency meters that monitor the line for the presence of high frequency signals. Such `palm of the hand`-held devices are customarily connected between tip and ring, and monitor whether the line is carrying signals that lie in a relatively high frequency band (above the normal audio band for voice signals). The devices may contain one or more LED display elements that are illuminated in association with the band of detected signals. The purpose of such devices is to alert the craftsperson to the presence of data traffic, so that the craftsperson will not proceed to connect a normal test set to the line and then cause the test set to go OFF-HOOK, if data traffic has been indicated.
To date, however, there is no mechanism, either integrated within or retrofitable to a craftsperson's test set, which actively prevents the craftsperson from even accidentally placing a line-connected test set in an OFF-HOOK condition, and thereby avoid the potential for severe degradation and/or interruption of data transmission on the line.